Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1.5–2

  Strong S winds over the past 36 hours have drifted and deposited new snow into locally deeper areas. Overnight, these areas of drifted snow have likely become more cohesive slabs. In some areas, especially across NW-N-NE aspects, these wind slabs deposited on top of a weak and sugary (faceted) old snow surface. Increasing slab cohesion + old snow weak layer = increased likelihood of triggering an avalanche.

  These winds slabs exist along the down wind side of ridgetops and in gully features, mainly in above treeline and near treeline terrain. Wind slabs may also exist in more isolated areas below treeline. 

  Use clues such as recent avalanches, drifting snow, cornice formation, new wind drifts, and snow surface cracking to identify areas of potentially unstable wind slab. Move around these areas with caution.

  

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Avalanches may also be triggered today in wind protected areas. This is most likely in locations where new snow has deposit on top of old, weak sugary snow. This old snow surface weak layer (sugary facets) is weakest on shaded, wind protected N aspects. It has been observed to some extent across many NW-N-NE aspects at a variety of elevations throughout the forecast area. In some areas, the most well-developed weak layer exists in mid-slope avalanche terrain or near the bottom of drainages.

  Pay attention to the amount of new snow that has accumulated in wind protected areas. A quick dig with your hand will reveal the old snow surface. Finding weaker, more clear looking, sugary snow underneath the new snow tells you that you are in an area where this weak layer exists. Snow surface cracking or recent avalanches are clues to the presence of this avalanche problem. Consider avoiding travel in avalanche terrain or below steep slopes in any area where signs of this avalanche problem exist.

  A couple of additional notes: 1) Significant amounts of graupel were noted yesterday in some locations, adding a weak layer within the storm snow. This may allow for shallow instability within the new snow to persist today on any aspect. 2) Rising snow levels today will put some rain on new snow. This could create some wet snow avalanches below about 7,000'. Wet loose or wet slab avalanches are possible today on a variety of aspects.